andygrove opened a new pull request, #2963:
URL: https://github.com/apache/datafusion-comet/pull/2963
## Which issue does this PR close?
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Closes #.
## Rationale for this change
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| Benchmark | Before | After | Improvement |
|--------------------------------|--------|--------|-------------|
| spark_lpad: default padding | 441 µs | 111 µs | -74.7% |
| spark_lpad: custom padding | 437 µs | 107 µs | -75.7% |
| spark_rpad: default padding | 443 µs | 108 µs | -75.4% |
| spark_rpad: custom padding | 445 µs | 107 µs | -75.8% |
| spark_lpad: multi-char padding | 336 µs | 158 µs | -52.3% |
| spark_rpad: multi-char padding | 337 µs | 145 µs | -56.1% |
| spark_lpad: with truncation | 178 µs | 82 µs | -54.7% |
| spark_rpad: with truncation | 173 µs | 79 µs | -54.0% |
## What changes are included in this PR?
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1. Reusable buffer: Added a String buffer that is cleared and reused for
each element instead of allocating a new String per element
2. Changed function signature: Replaced add_padding_string(String) ->
Result<String> with write_padded_string(&mut String, &str) that writes directly
to a buffer
3. Pre-computed pad characters: The pad_string.chars() iterator is now
collected once into a Vec<char> before the loop
4. Direct padding writes: New write_padding_chars() function writes
padding directly to the buffer without intermediate allocations, with a fast
path for single-character padding
5. Eliminated unnecessary allocations: Removed string.parse().unwrap()
which was converting &str to String unnecessarily
## How are these changes tested?
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